Depending on the internal state of an individual, the same sensory stimulus may be perceived differently at different times. Attentional processes are especially well known to allow an individual to filter out irrelevant stimuli and focus on aspects f the environment that are relevant for survival. What are the underlying neural mechanisms that contribute to this difference in perception and how do they influence behavior? In particular, the neural mechanisms underlying the influences of attention on odor processing are largely unknown and represent a major void in our understanding of olfactory system function. Human functional imaging research has however uncovered evidence that odor-directed attention modulates the representation of odors in the olfactory cortex - a finding which yields important implications for sensory system function. In addition, cholinergic influences on stimulus processing are hypothesized to provide a mechanism whereby attention enhances stimulus representation and the direct input of cholinergic fibers into the olfactory cortex by means of the horizontal diagonal band (HDB) provides an anatomical framework whereby attention-dependent cholinergic transmission may modulate odor coding in these structures. In the proposed research, I have developed two specific aims which will test the overall hypothesis that odor-directed attention controls odor signal-to-noise ratios in two prominent areas of the olfactory cortex which receive dense olfactory bulb input, the olfactory tubercle (OT) and the piriform cortex (PCX). Both Aims will involve recording neural activity from mice engaged in a powerful operant task designed to systematically manipulate odor-directed attention. In Aim 1, I will determine the impact of attention on the coding of odors among olfactory cortex neural ensembles. In Aim 2, I will employ optogenetics to directly test the influences of the HDB on odor coding and also attentionally-mediated odor coding among olfactory cortex neural ensembles. Overall, the results of this proposal will reveal fundamental principles whereby attention shapes the cortical coding of odor information in a manner whereby it may ultimately influence perception and odor-guided behaviors.